Embodiments of the invention relate generally to positron emission tomography (PET) and magnetic resonance (MR) imaging, and more specifically, to a combined PET-MR system incorporating a split bridge that provides patient support while reducing image degradation.
PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled agent is administered to a subject positioned within a detector ring. As the radionuclides decay, positively charged photons known as “positrons” are emitted therefrom. As these positrons travel through the tissues of the subject, they lose kinetic energy and ultimately collide with an electron, resulting in mutual annihilation. The positron annihilation results in a pair of oppositely-directed gamma rays being emitted at approximately 511 keV.
It is these gamma rays that are detected by the scintillators of the detector ring. When struck by a gamma ray, each scintillator illuminates, activating a photovoltaic component, such as a photodiode. The signals from the photovoltaics are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing deadtimes and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed into images depicting the distribution of the radionuclide-labeled agent and/or metabolites thereof in the subject.
MR imaging involves the use of magnetic fields and excitation pulses to detect the free induction decay of nuclei having net spins. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In combination PET-MR systems, a patient handling system is required in order to carry the patient in and out of the bore of the imaging system. The patient handling system includes a bridge positioned within the bore that extends through a length of the imaging system, with the bridge providing a path and support for a patient support pallet or bed that translates therealong to move the patient through the imaging system. While in a standalone MR system, the structure and mass of the bridge within the bore has no effect on image acquisition and image quality, such is not the case in a PET-MR system. That is, unlike in a standalone MR system, a PET-MR system requires minimum mass in the region of the PET detector in order to provide for optimum image acquisition. Specifically, the mass in the PET detector region attenuates gamma rays, which reduces PET signal to the detectors and degrades image quality (IQ).
It would therefore be desirable to provide a bridge for use in a PET-MR system that helps in reducing the degradation of image quality. It would also be desirable for the bridge to provide improved image quality without compromising on patient support functionalities and serviceability.